When a camera is used to record optical images and objects relatively far from the camera lens are in focus, objects nearer to the camera lens are out of focus and their recorded images are blurred. On the other hand, when focusing on the nearer objects, the farther objects are out of focus and their recorded images are blurred. This problem is believed to be related to the intrinsic characteristics of the optical system. According to a basic optical principle, a pinhole lens has an infinite focal depth, and all objects are in focus when their images are recorded within the pinhole lens of a pinhole camera. However, for the ideal! pinhole lens, the hole is so small that not even a single photon can pass through it, and hence no image is formed. That is, it is merely a hypothetical lens.
Due to this basic physical limitation, the lens has a finite focal depth, and the picture of the object is taken by means of automatic or manual focusing. However, when two objects, one nearer the camera and the other farther away from it, are to be photographed in the same picture, it is impossible to bring both into focus at the same time. That is, while bringing one object into focus, the other goes out of focus and its image becomes blurred. This is a disadvantage.
Although not a technology for forming a synthesized image, a technology known as inverse filtering has been tested which attempts to create a clear image from the blurred portion of a picked-up image by means of enhancing the blurred portion. However, since it also amplifies noise, it fails to reach the stage of practical applications.